Photolithography or microlithography apparatus are widely used in the fabrication of microelectronic semiconductor devices and other microdevices. In photolithography, an optical system directs light energy to record a pattern at high resolution and with precise registration onto a photosensitive layer formed on a silicon wafer or other substrate. Continuing improvements in miniaturization place increasingly more challenging demands on the performance and accuracy of the optical system used for this function. Microlithography optical systems are fairly large and complex, containing a number of optical elements.
Achieving correct magnification and focus are critical for obtaining precise layer-to-layer registration and submicron resolution with photolithographic optics used for device fabrication. For example, in order to properly adjust magnification or focus, it is often necessary to move specific components of the optical system to specific positions along the optical axis. In obtaining this movement, it is important to minimize or eliminate inadvertent movement of other components of the optical system. For example, focus adjustment is usually enabled by displacement of an optical element along the optical axis, conventionally the z-axis, with no translation in the orthogonal x or y axes.
In conventional camera optics, focusing is often accomplished using a threaded mount mechanism. However, even with precision machining, it is extremely difficult to achieve, with threaded fittings, the level of positional accuracy needed to prevent unintended shifting of components perpendicular to the optical axis. As a general rule, passive mechanical fittings or sliding components of this type can tend to exhibit additive and non-repeatable tolerance errors. The relative size and complexity of photolithography optics further compound this problem. Because of this, threaded fittings are generally not well-suited for providing focus adjustment with high-precision semiconductor microlithography optics. Instead, a stacked annuli lens assembly arrangement is preferred for this type of optical apparatus, as described, for example, in U.S. Pat. No. 5,428,482 entitled “Decoupled Mount for Optical Element and Stacked Annuli Assembly” to Bruning et al.
Where lens axial adjustment may be necessary in a stacked annuli arrangement, solutions that take advantage of balanced or kinematic constraining forces, using springs and flexures for example, can be more promising for high precision adjustment applications than are static solutions. However, proposed solutions of this type for providing pure axial translation adjustment are typically highly complex, often requiring precision fabrication and assembly of multiple interconnecting parts. As just one example, in the embodiment described in U.S. Pat. No. 6,538,829 entitled “Optical Element Mount Comprising an Optical Element Holding Frame” to Rau et al., an optical mount for adjusting two components relative to each other is shown. The mechanism described in the '829 Rau et al. disclosure employs a fairly complex network of flexures and hinges for providing this type of axial translation adjustment.
Radial flexures have been used for accurate axial positioning in optical applications. For example, U.S. Patent Application Publication 2006/0001886 entitled “Precision Retroreflector Positioning Apparatus” by Zacharie et al. describes retroreflector mounting for an interferometer using an arrangement with radial flexures to provide minimal axial deviation for this device. While this type of approach has inherent mechanical advantages, the part count when using this type of solution is sizable and problems with additive tolerances can occur.
It is known in the field of optical design and precision mechanics that flexures can be used to connect two bodies in order to define certain patterns of constraints, thereby allowing certain desired degrees of freedom (DOF) between the two bodies, while suppressing or inhibiting others. One pattern that allows a single DOF of straight-line motion is an arrangement of two sets of three constraints, where the constraints of each set are coplanar, and the planes defined by each set are parallel and are separated from each other by some distance. Such a pattern of constraints results in a single degree of freedom of translational motion along a line that is perpendicular to the planes of the constraints. It is also well known that in order to achieve purity of motion, the three constraints of each set should be arranged in a trilaterally symmetric pattern; each constraint tangent to a circle. The line joining the centers of the two circles defined by these constraints should be perpendicular to the two planes.
Referring to FIG. 1A, there is shown an inner member, lens holder 10, in a frame 12, with reference xyz axes designations. The z axis corresponds to the optical axis O. For this application, only movement parallel to the z-axis is desirable. Translation in the x-y plane or rotation about x or y axes (θx, θy rotation) must be prevented. For spherical optics, rotation about the z axis would, in general, not be objectionable; however, there must be no rotation about the x or y axes. The mount mechanism that connects lens holder 10 to frame 12 must allow translational motion only in the direction of optical axis O, that is, the z-direction.
Flexures have been used to provide the needed mechanical constraint in lens applications. For example, FIG. 1B shows a lens mount 20 that provides this single degree of freedom, z-axis translation along optical axis O. Lens holder 10 is suspended from frame 12 by means of two parallel sheet flexures 14a and 14b. Parallel sheet flexures 14a and 14b lie in two parallel planes, Pt and Pb. Sheet flexure 14a lies in top plane Pt. Bottom sheet flexure 14b lies in bottom plane Pb. Each sheet flexure 14a, 14b has spokes 16 that extend from frame 12 to lens holder 10 and provide a linear constraint. The combination of these symmetric spokes 16 with sheet flexures in the parallel planes provided allows a single translational degree of freedom, along optical axis O.
FIG. 1C shows an alternative configuration of lens mount 20 that applies the same constraint pattern shown in FIG. 1B but employs folded sheet flexures, termed fold flexures 18 in the context of this application. Again, lens holder 10 is suspended from frame 12 by three pairs of fold flexures 18. Each fold flexure 18 provides a single constraint along its fold 32. In the context of this application, a reference to a fold can be considered with respect to the inner fold that is formed at the juncture of inner surfaces of the flexure.
As is shown in FIG. 1D, the set of top inner folds 32t lie in top plane Pt; the set of bottom inner folds 32b lie in bottom plane Pb, again with planes Pt and Pb parallel to one another. This relationship is important for allowing translation only along the optical axis O. Kinematically considered, if planes Pt and Pb were not parallel, there would be some freedom of movement relative to the x-y axes.
Using conventional designs, top and bottom fold flexures 18 are paired in such a way that they are radially aligned or overlapping with respect to a view taken along optical axis O. That is, as shown in FIG. 1E, with respect to a view taken along optical axis O, top and bottom fold flexures 18 both extend radially outward from axis O at the same angles. In the particular example of FIG. 1E, the pair of top and bottom fold flexures 18 shown at the phantom line are radially aligned or overlapping, both at angle φ with respect to a common reference. Their respective folds lie along parallel lines and may be overlapping or aligned, so that there are pairs of overlapping folds with respect to this view.
While the arrangement of fold flexures 18 shown in FIG. 1C has advantages for control of lens motion, however, there are still problems that prevent its use for high-precision photolithography applications. Even the slightest parasitic effects or asymmetries of construction can compromise the purity of motion demanded for lens adjustment in high-resolution photolithography. Materials used for fold flexures 18 and their fasteners must be carefully specified to minimize thermal effects due to differences in coefficients of thermal expansion. Precision machining and assembly techniques are required for attaching fold flexures 18 to both lens holder 10 and frame 12. In light of these problems, plus considering the additive tolerance errors inherent to conventional fabrication and assembly techniques, the solution of FIG. 1C has proved impractical for high-precision photolithography applications.
Overall, conventional lens mounting methods are likely to cause overconstraint and other problems that limit their usefulness for photolithography applications. While various solutions for axial positioning have been proposed, there remains a need for an optical assembly mount that allows adjustment of optical components along the optical axis, but inhibits movement along or about axes other than the optical axis, uses a minimum number of parts, and provides the level of performance necessary for use with optical assemblies for microlithography and other precision optical and positioning applications.